Paste nickel electrode plate and a storage battery including an electroconductive material

ABSTRACT

A paste nickel electrode plate comprising a porous metal substrate filled with a mixture of a nickel hydroxide powder used as an active material and a CoO powder used as an electroconductive material wherein the CoO powder has a covering layer of hydroxyl groups on the surfaces of the powder particles to prevent the formation of Co 3  O 4  on the surfaces of the CoO powder particles. A storage battery is provided using the above-described paste nickel electrode plate as a positive electrode.

FIELD OF THE INVENTION

The present invention relates to a paste nickel electrode plate used ina storage battery, a storage battery and an electroconductive materialtherefor.

BACKGROUND OF THE INVENTION

Sintered and paste positive nickel electrodes for use in an alkalinestorage battery, such as a nickel-hydrogen battery, a nickel-cadmiumbattery and the like, are known. The paste electrodes in particular havedrawn attention since it is possible to provide a high-capacity batteryusing paste electrodes.

However, as compared with sintered nickel electrode plates, paste nickelelectrode plates are less advantageous with respect to their coefficientof utilization and rapid discharge characteristics. To increase theconductivity and coefficient of utilization while minimizing thepolarization of the paste electrodes, a paste nickel electrode plate hasbeen proposed wherein during manufacture a nickel (Ni) powder is addedto a cobalt oxide (COO) powder or the like to form an electroconductivematerial which is mixed with an aqueous thickener solution. This mixtureis kneaded together to form a paste. A porous metal base plate orsubstrate is filled with the paste and the pasted substrate then driedand roll-pressed.

It has been found that when a CoO powder is used as an electroconductivematerial, the coefficient of utilization of the electrode is lowered dueto the formation of Co₃ O₄ on the surfaces of the particles of the CoOpowder. Thus, CoO powder conventionally is manufactured in a processwherein a Co compound, such as cobalt hydroxide, is burned or baked in anitrogen gas atmosphere to form CoO. The resultant CoO powder is thencooled with nitrogen gas. However, if the CoO powder is left in theambient atmosphere for a long period of time or if a paste electrodeplate containing the CoO powder is exposed to the ambient atmosphere fora long period of time, the surfaces of the CoO powder are oxidized andCo₃ O₄ is formed thereon. Accordingly, when CoO powder manufactured byconventional methods is used as an electroconductive material in a pastenickel electrode, a lower coefficient of utilization results.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides a paste nickel electrode plate for use ina storage battery having an improved coefficient of utilization.According to this invention a paste nickel electrode plate is preparedby filling a porous metal substrate with a mixture comprising a nickelhydroxide powder as an active material, and a CoO powder as anelectroconductive material wherein the CoO powder has a covering layerof hydroxyl groups on the particle surfaces. Since the CoO powder has acovering layer of hydroxyl (OH) groups formed on the powder particlesurfaces, the CoO portion of the powder particles is protected fromexposure to the ambient atmosphere and oxygen dissolved in an alkalineelectrolyte solution used in a storage battery. The CoO, therefore,remains active. The manner in which the covering layer of hydroxylgroups is provided on the surfaces of the CoO powder particles isdescribed hereinafter.

When a paste nickel electrode plate containing the CoO powder protectedwith hydroxyl groups is used as a positive electrode in an alkalinestorage battery, the CoO powder is changed to HCoO₂ ⁻ ions in analkaline electrolyte solution. The diffusion of the HCoO₂ ⁻ ions in thepositive electrode can be carried out satisfactorily so that a uniformelectroconductivity throughout the electrode plate is assured therebybringing about an improved coefficient of utilization of the positiveelectrode resulting in an increased battery capacity and improved rapiddischarge characteristics.

DETAILED DESCRIPTION AND PRESENTLY PREFERRED EMBODIMENTS

A preferred embodiment of an electroconductive material according to thepresent invention is provided as follows. A cobalt compound, generallyany cobalt salt, cobalt oxide or the like, is used as a raw material andthis raw material introduced into a baking furnace. An inert gas, suchas nitrogen gas, is also introduced into the furnace and the cobaltcompound heated at a high temperature, preferably at 900° C. or higher,in the inert gas atmosphere for a length of time sufficient to carry outthermal decomposition of the cobalt compound and obtain CoO powderhaving a purity of 98 wt. % or higher. Immediately after the CoO isformed or after the CoO formed has been cooled to room temperature whilein the inert gas atmosphere, steam is introduced into the furnace tomake the inert gas atmosphere therein appropriately moist, i.e.,preferably the inert gas atmosphere is provided with a humidity of about60%, and the CoO maintained in the atmosphere containing moisture andinert gas. Thereafter, the CoO powder at room temperature is removedfrom the furnace. In this manner, CoO powder is provided with a coveringlayer of OH groups on the surfaces of the powder particles due to thereaction of the water molecules from the moisture present in the inertatmosphere with the CoO powder particles. Thus, a CoO powder is producedhaving a covering or protective layer of OH groups. It is preferablethat CoO powder according to the present invention is treatedsufficiently so that the CoO powder has about 90% or more of the surfacearea of its particles covered with the protective layer of OH groups.

The covering layer of OH groups prevents the formation of an oxide of ahigher order, such as Co₃ 0₄ or the like, on the powder even when thepowder is contained in a positive electrode plate as anelectroconductive material and left exposed to the ambient atmosphere,or the positive electrode is brought into contact with oxygen dissolvedin an electrolyte solution. As a result, the CoO powder is protectedfrom the potential deterioration of the powder's electroconductivity andactivity. This insures stable performance with respect to theelectroconductivity and activity of the material over a long period oftime. The CoO powder according to the present invention, therefore, isparticularly useful for mixing with nickel hydroxide in themanufacturing of a paste nickel electrode plate. The coefficient ofutilization of the resultant positive electrode is improved. This isshown by the examples set forth below.

EXAMPLE 1

A commercially available cobalt hydroxide [Co(OH)₂ ] powder was placedin a treatment container and nitrogen gas introduced into the container.The Co(OH)₂ powder was heated at 900° C. for 15 hours in the nitrogengas atmosphere to thermally decompose the Co(OH)₂ powder into CoOpowder. Thereafter, the CoO powder was cooled to room temperature in thesame inert gas atmosphere. The CoO powder was then treated at 25° C. ina nitrogen gas atmosphere having a humidity of 60% so as to form on thesurfaces of the particles a covering layer of OH groups. The treated CoOpowder was removed from the treatment container. The formation of thelayer of OH groups was confirmed by means of x-ray photoelectronspectroscopy (XPS).

COMPARISON EXAMPLE

For comparison, the same commercially available Co(OH)₂ powder was usedto form a CoO powder using the same thermal decomposition procedure asdescribed in Example 1. The CoO powder was left exposed to the ambientatmosphere for six months whereby a covering layer of Co₃ O₄ formed onthe surfaces of the particles of CoO. The formation of the Co₃ O₄ layerwas confirmed by means of XPS.

The CoO powder with the OH group covering layer as manufactured inExample 1, and a CoO powder having a Co₃ O₄ covering layer were eachused as an electroconductive material in a small amount in mixture witha nickel hydroxide powder. Each mixture was combined with an aqueousthickener solution to form a paste. A porous metal substrate was filledwith each paste in a conventional manner to manufacture a paste nickelelectrode plate according to the present invention and a comparativepaste nickel electrode plate. The plates were then used in aconventional manufacturing method to produce storage batteries whichwere compared with each other.

More particularly, five grams of the CoO powder with the protective OHlayer and five grams of the comparative CoO powder having a Co₃ O₄covering layer were mixed, respectively, with ten grams of acommercially available Ni powder, as sold under the tradename INCO #255,and 85 grams of a commercially available spherical nickel hydroxidepowder. Each resultant mixture was combined with 35 grams of a 1%aqueous solution of carboxymethylcellulose (CMC) and kneaded to obtain apaste. Each of the respective pastes was applied to a porous metal baseplate. In this example, a foam nickel substrate was used although afibrous nickel felt substrate or other conventional substrate can alsobe used. Each paste was applied to the substrate so as to fill the poresof the substrate with the paste. Thereafter, each pasted substrate wasdried, sprayed with a PTFE disperse liquid, and roll-pressed to apredetermined thickness to provide a paste nickel electrode plateaccording to the present invention and a comparative paste nickelelectrode plate. The respective paste nickel electrode plates of theexamples were each prepared to have a theoretical capacity of 1180milliampere-hour (mAh).

Both the paste nickel electrode plate according to the present inventionand the comparative paste nickel electrode plate manufactured asdescribed above were used as a positive electrode, paired with anegative electrode made of a hydrogen-occlusion alloy in a stackedlaminate fashion with a separator interposed therebetween, and rolledinto a spiral form to obtain spiral battery elements. Each spiralbattery element was inserted into a cylindrical container of the samedimensions. The containers were covered and sealed as soon as analkaline electrolyte solution having an identical specific gravity of1.36 was poured into each container. In this manner, an 1100 mAh sealednickel-hydrogen battery according to the present invention and acomparative counter-part sealed nickel-hydrogen battery weremanufactured.

The batteries were charged with a 0.2 Coulomb (C) current at an ambienttemperature of 20° C. for 7.5 hours and discharged with the same currentto a final battery voltage of 1.0 Volt (V). This charge-dischargeoperation was repeated twice and, thereafter, the batteries were left at40° C. for 24 hours and subjected to an initial activation process.After the initial activation process, a capacity test and rapiddischarge test as described below were conducted.

In the capacity test, the batteries were charged with 0.2 C current at20° C. for 7.5 hours and discharged with 0.2 C current to a finalbattery voltage of 1.0 V. The battery capacity was then measured todetermine the coefficient of utilization of each paste nickel electrode.

In the rapid discharge test, the batteries were charged with 0.2 Ccurrent at 20° C. for 7.5 hours and, thereafter, discharged with 3 Ccurrent to a final battery voltage of 1.0 V to measure the rapiddischarge capacity and determine the coefficient of utilization of eachpaste nickel electrode.

The test results obtained are set forth in Table 1 below.

                                      TABLE 1                                     __________________________________________________________________________                     COEFFICIENT                                                                            RAPID   COEFFICIENT                                           BATTERY                                                                              OF       DISCHARGE                                                                             OF                                                    CAPACITY                                                                             UTILIZATION                                                                            CAPACITY                                                                              UTILIZATION                                           (mAh)  (%)      (mAh)   (%)                                         __________________________________________________________________________    BATTERY   1227   104.0    1027    87.0                                        OF THE                                                                        PRESENT                                                                       INVENTION                                                                     COMPARATIVE                                                                             1156   98.0      968    82.0                                        BATTERY                                                                       __________________________________________________________________________

As clear from Table 1, the battery of the present invention showsremarkable improvement over the comparative battery with respect to eachof the battery capacity and its coefficient of utilization, and therapid discharge capacity and its coefficient of utilization.

Accordingly, a paste nickel electrode plate according to the presentinvention containing a CoO powder with a OH group protective layerformed on the surfaces of the powder particles is useful as anelectroconductive material. The potential oxidation of the CoO powderparticles is thereby prevented and a highly satisfactory and stableactivity of the electrode maintained. Therefore, when the paste nickelelectrode of the invention is used as a positive electrode in analkaline storage battery, an improved coefficient of utilization resultswhich leads to improvements with respect to battery capacity and rapiddischarge characteristics.

As will be apparent to one skilled in the art, various modifications canbe made within the scope of the aforesaid description. Suchmodifications being within the ability of one skilled in the art form apart of the present invention and are embraced by the appended claims.

It is claimed:
 1. A paste nickel electrode plate for a storage battery,comprising a porous metal substrate having the pores thereof filled witha mixture comprising a nickel hydroxide powder and a cobalt oxidepowder, said cobalt oxide powder having a covering layer of Co(OH)₂ onthe surface of the cobalt oxide powder particles.
 2. A storage batterycomprising a positive electrode, a negative electrode, an electrodeseparator, an alkaline electrolyte solution and a container wherein thepositive electrode is the electrode plate of claim
 1. 3. A paste nickelelectrode plate according to claim 1 wherein said covering layer ofCo(OH)₂ covers at least about 90% of the surface area of the cobaltoxide powder particles.
 4. A method of manufacturing anelectroconductive material comprising subjecting a cobalt compound tothermal decomposition in an atmosphere containing an inert gas to formcobalt oxide powder, and maintaining the cobalt oxide powder formed in amoist atmosphere containing an inert gas until a covering layer ofCo(OH)₂ is formed on the cobalt oxide powder particle surfaces.
 5. Amethod according to claim 4 wherein said cobalt compound is a cobaltsalt or cobalt oxide.
 6. A method according to claim 4 wherein saidinert gas is nitrogen.
 7. A method according to claim 4 wherein saidmoist atmosphere containing an inert gas is provided by injecting steaminto the atmosphere containing an inert gas.
 8. A method according toclaim 4 wherein said covering layer of Co(OH)₂ covers at least about 90%of the surface area of the powder particles.
 9. An electroconductivematerial prepared according to the process claimed in claim 4.